Had a call from Intel this morning, and a long conversation.
We concluded that there's probably an incompatibility between the MoBo BIOS and SSDs in general. He was interested that the "power delay" would make it work, and suspects there's a toggle to do that somewhere in the MoBo BIOS (but doesn't know, and they aren't allowed to contact them to find out).
I'll have a sniff around in the BIOS, then do my thing with the soldering iron.
Getting the "Drive C" off the existing partitioned hard drive onto the SSD and getting it to boot could be "interesting". His advice was to move all the other partitions to a different HDD, then clear the present boot drive down to less than 120GB, then use the Intel migration tool.
A bit of a pain, but may work. I'll probably try the EASEUS/EasyBCD thing first since it's a lot easier.